Transcript Lecture - Faculty

Galileans to Scale
Interiors of the Galileans
Overview of Surfaces
Outer Galilean Moons
Callisto:
• Moderately dark
surface
• Heavily cratered
• No magnetic field
• ~1/4 of interior is ice
(by mass)
Ganymede:
• Moderately bright
surface
• Weak magnetic field
and evidence for a
denser core
• Perhaps melted in
past to allow for
differentiation
Callisto Terrain
Ganymede Terrain
Inner Galilean Moons
Europa:
• Bright surface
• Absence of craters –
young surface
• Possible mag. Field and
denser core
• Surface ice is a thin layer
(100’s of meters) floating
on H2O ocean (!) –
requires a source of
heating
Io:
• Rocky interior
• Orbits inside Jupiter’s
magnetosphere
• Io is tidally heated and
volcanically active
(predicted in 1979 and
discovered by Voyager 1
in same year)
• Surrounded by a yellow
Na-cloud
Europa Terrain
Crater on Europa
Close-up on Europa
Subsurface Model
IO: Example Volcanic Activity
Volcanic Eruption at Io:
Before and After
Io’s Sodium Cloud
Sulphur Ring of Io
Titan
• 1944 – Kuiper discovers a thick atmosphere
• Atmosphere is a reddish, featureless haze; mainly N2
with 10% CH4 and smog:
• Psurf=1.6PE and Tair=93 K (-292oF)
• Only satellite with a thick atm.:
– CH4 rain, snow, and ice?
– Pools of liquid N2 and CH4
• 2004 – Cassini probe called Huygens parachuted into
Titan’s atmosphere
Titan to Scale
Impression of View of Saturn
from above Titan
Surface of Titan
Earth-Titan Comparison
The Active Atmosphere of Titan
Huygens Probe at Titan
Cold Geysers
at Enceladus
An Ocean Below Enceladus’ Icy Crust?
• NASA’s Cassini spacecraft has
observed plumes of material
escaping from Saturn’s small
icy moon, Enceladus
• The plume is mostly water
vapor, with tiny ice particles and
other gaseous molecules mixed
in (e.g. CO2, CH4, C2H6)
• The plume supplies ice particles
to one of Saturn’s rings
• Some ice particles contain salt,
which may indicate they
originate in an ocean deep
below the icy crust
Image mosaic of Enceladus taken by Cassini,
showing individual plumes of gas and ice escaping
from the surface. The plumes extend 100’s of km
into space from the ~500 km diameter moon.
What Process Creates the Plume?
• Plumes may be material
escaping through surface cracks
from an internal salty ocean or
lake
• Alternatively, ice along cracks
may sublime or melt, followed by
escape of water vapor and icy
particles
• Many scientists find the salty
ocean model most convincing,
but others favor combinations of
alternative explanations
Left: Enceladus may have a salty subsurface
ocean that releases material to space through
cracks in the moon’s icy shell. Right: The
walls of icy cracks in the surface may melt or
sublime, venting gas and icy particles to space.
The Big Picture
• Enceladus is surprisingly active for
such a small body - likely a
consequence of tidal heating
• Future flybys of Enceladus by
Cassini may help to resolve whether
Enceladus joins the growing “club”
of solar system bodies believed to
have oceans
• If Enceladus has an ocean, then it
contains all of the ‘‘ingredients’’
known to be important for life: liquid
water, molecular building blocks,
and energy
Tiger stripes
Image of Enceladus showing the
‘tiger stripes’ region in the southern
hemisphere, where the plumes
originate
Mimas
The Impact at Mimas
Diameter ~ 390 km
Huge crater from impact, almost big enough to shatter this moon
Estimate:
m
M
v
Binding Energy of Mimas
3 GM 2
E»
5 R
Kinetic Energy of Collider
1 2
K » mv esc
2
Mass and Size of Collider
m » 3 ´1017 kg » 10-5 M
radius » 4 km (if r = 1000kg/m3 )
rH2O
D
Moons of Uranus
Umbriel and Oberon
• Heavily cratered
• Large cracks
Ariel and Titania
• Lighter cratering
• Resurfacing by flows of volcanic water (!)
• Some regions of Ariel are featureless - very recent!
• System of deep cracks (tidal heating effects?)
Miranda
• Rolling and cratered terrain
• Some fractures
• 20 km cliff - takes 10 min to fall (in contrast to 1 min as at Earth)
Moons of Uranus
Miranda
Triton: Geyser Plumes
Triton
• Triton is the only large moon with a retrograde orbit
(possibly a captured body?)
• Tidal bulges on Neptune lag instead of lead Triton
 Orbit is degrading
 Will eventually shatter as nears Roche Limit (in ~109
years to make a ring to rival Saturn’s)
• Composition: 75% rock, 25% ice
• Atm.: thin N2
• Polar Capts: N and CH4 ices
• Evidence for resurfacing and internal activity, even
geyser eruptions of N gas, leading to sooty plumes
The Tides at Triton